Consolidate documentation (#3071)

* 🔥 consolidate documentation
* ♻️ overwork std specializations
* 🚚 move images files to mkdocs
* ♻️ fix URLs
* 🔧 tweak MkDocs configuration
* 🔧 add namespaces
* 📝 document deprecations
* 📝 document documentation generation
* 🚸 improve search
* 🚸 add examples
* 🚧 start adding documentation for macros
* 📝 add note for https://github.com/nlohmann/json/issues/874#issuecomment-1001699139
* 📝 overwork example handling
* 📝 fix Markdown tables

doc/README.md

@@ -0,0 +1,20 @@
+# Documentation
+
+## Generate documentation
+
+Note on documentation: The source files contain links to the online documentation at https://json.nlohmann.me. This URL
+contains the most recent documentation and should also be applicable to previous versions; documentation for deprecated
+functions is not removed, but marked deprecated.
+
+If you want to see the documentation for a specific tag or commit hash, you can generate it as follows (here for tag
+`v3.10.2`):
+
+```shell
+git clone https://github.com/nlohmann/json.git
+cd json
+git checkout v3.10.2
+make install_venv serve -C doc/mkdocs
+```
+
+Open URL <http://127.0.0.1:8000/> in your browser. Replace from any URL from the source code `https://json.nlohmann.me`
+with `http://127.0.0.1:8000` to see the documentation for your tag or commit hash.